1. Field of the Invention
The present invention relates to an electronic control unit for an internal combustion engine that identifies cylinders and detects crank angles from signals issued by a crank angle sensor.
2. Description of the Related Art
A signal in synchronization with the revolution of an engine is used to control the ignition timing, fuel injection, etc., of an internal combustion engine. A generator producing the signal usually detects the revolution of a camshaft or a crankshaft of the engine. An example of a crank angle sensor is shown in FIG. 5 and FIG. 6. The crank angle sensor shown in these figures includes a rotary shaft 1 being rotatable in synchronization with an engine (not shown), a rotary disc 2 mounted on the rotary shaft and provided with a window 3 at a location corresponding to a desired detection angle, a light emitting diode 4, a photodiode 5 for receiving the light emitted from the light emitting diode 4, an amplifier circuit 6 connected to the photodiode 5 for amplifying an output signal of the photodiode 5, and an output transistor 7 connected to the amplifier circuit 6 and having an open collector. A window 3' for identifying a particular cylinder is provided in the rotary disc 2 so that it is asymmetrical to the window 3 so as to identify another cylinder.
Thus, the crank angle sensor outputs a signal illustrated in FIG. 7. The signal indicates that the falling edge of the signal for a particular cylinder, namely, cylinder #1, is offset 10 degrees toward delay side (ATDC 5 degrees or 5 degrees after the top dead center) from other cylinders, namely, cylinder #2, cylinder #3, and cylinder #4. The rising edge of the signal for all the cylinders is BTDC 75 degrees or 75 degrees before top dead center.
Referring now to FIG. 8 and FIG. 9, the operation for identifying a particular cylinder will be described. As shown in FIG. 8, the output signal of a crank angle sensor 8 is supplied to a microcomputer 10 via an interface circuit 9. The microcomputer 10 identifies the cylinder according to a flowchart shown in FIG. 9. First, in step S1, a high-level output period t and its rising section cycle T of a signal waveform shown in FIG. 7 are calculated. Then, the sequence proceeds to step S2 wherein a ratio t/T is calculated. Subsequently, in step S3, a mean threshold value .alpha.n that gives t1/T&gt;.alpha.&gt;t0/T is provided, and .alpha.n is determined according to the following operational expression: EQU .alpha.n=(1-k).alpha.n-1+k(t/T)n where k=a constant
The value of .alpha.n calculated in step S3 is compared with the ratio t/T (step S4), and if t/T-.alpha.n&gt;0, then it is discriminated that the cylinder is the particular cylinder and an identification flag is set (step S5). If it is found in step S4 that t/T-.alpha.n&lt;0, then it is determined that the cylinder is a different (i.e., non-particular) cylinder.
In the conventional cylinder identification apparatus for an internal combustion engine, if the duty of a signal in relation to the output signal cycle corresponding to a cylinder that is obtained from the crank angle sensor should indicate an abnormal value due to a fluctuating revolution of the engine or the like, then this is undesirably reflected on the threshold value .alpha.n, posing a problem in that, even after the duty of the signal restores a normal value, the influence by the foregoing abnormal value stays on, resulting in erroneous determination.
A solution to the aforesaid problem has been proposed in Japanese Examined Patent Publication No. 6-84739. The second conventional prior art example is designed so that, if an abnormal signal duty occurs, then it is inhibited from affecting the calculation of a threshold value to thereby permit quick and accurate identification of a cylinder.
FIG. 10 is a simplified block diagram of a cylinder identification apparatus for an internal combustion engine in accordance with the conventional art, and FIG. 11 is a flowchart illustrating a cylinder identification routine effected by the cylinder identification apparatus.
The cylinder identification apparatus shown in FIG. 10 includes the same components 8 and 9 as those of the first prior art example, a microcomputer 10A, an operational calculator or calculating means 11, a comparator or comparing means 12, and a cylinder identifier or cylinder identifying means 13.
Referring to the flowchart of FIG. 11, the cylinder identification operation performed by the cylinder identification apparatus for an internal combustion engine configured as discussed above will be described. The microcomputer 10A shown in FIG. 10 calculates, by the calculating means 11, the values of a high-level output period t and its rising section cycle T of a signal sent from the crank angle sensor 8 via the interface circuit 9 according to FIG. 2 (step S1).
Then in step S2, by using the calculating means 11, a ratio t/T is calculated on each cylinder by adopting t and T, which have been determined in step S1. In step S6, it is determined by the comparing means 12 whether or not the ratio t/T lies within a specified range; if the t/T has been found to lie outside the specified range, then the present ratio t/T is set to a specified value in step S7. In other words, step S7 involves a calculation inhibiting means for inhibiting the value of the present ratio t/T from being reflected in the calculation of the threshold value .alpha.n if the value of the present ratio t/T turns out to be abnormal. The microcomputer 10A then advances to step S3 wherein it causes the calculating means 12 to calculate a mean threshold value [.alpha.n=(1-k).alpha.n-1+k(t/T)n] that is the mean value of the ratios of all cylinders based on the specified value.
On the other hand, if the value of t/T obtained in step S6 is found to lie within the specified range, then the foregoing mean threshold value .alpha.n is calculated based on the value of the present ratio t/T by the calculating means 12. In step S4 and step S5, the same operation as that in the conventional art is carried out, and the value of the ratio for each cylinder, which has been obtained as mentioned above, is compared with the mean threshold value .alpha.n (step S4), and if the comparison result indicates that the former is larger than the latter, then the cylinder is identified as a particular cylinder. In this case, cylinder #1 is identified and a flag is set at a register corresponding to cylinder #1 (step S5). If the comparison result indicates that the former is smaller than the latter, then the process returns. Thus, when a particular cylinder or cylinder #1 in this case has been identified, revolution signals will be obtained in the order of cylinder #1, cylinder #3, cylinder #4, and cylinder #2, so that the remaining cylinders can be identified in the order of cylinder #3, cylinder #4, and cylinder #2.
The cylinder identification technique discussed above enables accurate determination even if the signal duty should have an abnormal value due to fluctuations in revolution of an engine since the abnormal value is not reflected on the calculation of the mean threshold value.
The second conventional art, however, has the following shortcoming.
If an engine is started while it is cold or with a deteriorated battery or the like, marked cyclic variation is observed in the revolution of the engine. This may sometimes lead to cylinder identification errors, resulting in improper ignition, erroneous fuel injection, or other improper engine control.
Also if a hastened engine start is made or if hastened engine start is repeated, marked cyclic fluctuations in revolution may take place or the revolution of an engine may be reversed. This is also likely to lead to cylinder identification errors with a resultant possibility of erroneous engine control.
A plurality of crank angle signals are detected to identify cylinders before starting ignition control or fuel injection control, thus taking a prolonged time to complete a startup process.
For the crank angle sensor 5, there has been known a magnetic type sensor as disclosed in Japanese Examined Patent Publication No. 7-81547. In this type of crank angle sensor, teeth composed of projections and recessions are formed on the outer periphery of a round rotary magnetic member, which is provided on a camshaft or the like that rotates in synchronization with the crankshaft, and the rotational positions of the teeth are magnetically detected by using a magnetic sensor. Using the magnetic crank angle sensor poses a shortcoming in that even if inaccurate detection is carried out due to a failure of the crank angle sensor such as a missing tooth of its rotary magnetic member, the failure of the crank angle sensor cannot be detected. This also causes a cylinder identification error, resulting in a possibility of engine control errors, including erroneous ignition and erroneous fuel injection.